Polyesters mainly consisting of polyethylene terephthalate having a high degree of crystallinity and a high softening point are widely used in various commercial fields, because they have excellent properties such as stretchability, flexural strength, chemical resistance, light resistance, heat resistance and the like. A polyester may be formed into a film depending upon the intended service of the products. However, it is known that such a film has deficiencies in that the film is easily electrostatically charged and abraded by guide members and hand slipperiness is insufficient. It is also known that these troubles can be eliminated by improving the slipperiness of the polyester film. Proposals have been made to employ antistatic agents or lubricants to improve the slipperiness. However, these methods cannot be easily utilized because of reduced electrical properties, low operation efficiency, and increased cost.
While various methods for improving the slipperiness have been proposed, the most usual is to distribute fine particles of an inert material at the surface of a film. In general, fine particles can be incorporated by the following two methods.
In the first, a precipitation method, a calcium compound catalyst which is used in an ester-interchange reaction is precipitated during the polycondensation reaction in the form of fine particles of calcium terephthalate and a calcium salt of polyester oligomer (both of which are insoluble in the polyester). Terephthalic acid may be added to the reaction system to facilitate the precipitation of fine particles. In still another method, terephthalic acid and calcium acetate may be added at the stage of polycondensation to precipitate calcium terephthalate particles. In all of these cases, fine particles are precipitated from within the reaction system.
A second method is called the addition method. Silica, kaolin, talc or the like, after pulverization if necessary, is added either to a polyester-forming reaction system or to a polyester compound before molding.
Calcium terephthalate as such can be formed in a polyester according to the above-described precipitation method. The calcium terephthalate particles precipitated the polyester may be used to improve the slipperiness. It is, however, difficult to control the degree of slipperiness. In addition, the calcium terephthalate precipitated in the polyester are inclined to be broken during stretching in the course of film formation and consequently a slipperiness of the formed film will be far below the level expected from a calcium terephthalate containing polyester before the film formation.
The film is required to have abrasion resistance as well as slipperiness. Particularly when the film is used as a magnetic tape, the abrasion of the film by magnetic heads and other guide members should be small. Less satisfactory properties result from the prior art precipitation method making use of fine particles of calcium terephthalate precipitated in the polyester.